Near field RFID probe with tunning

ABSTRACT

A near field radio-frequency identification (“RFID”) probe includes a probe tip comprising a resonant coil configured to communicate with an RFID compatible device at a predetermined resonant frequency. The near field RFID probe further includes a plurality of switch capacitor networks each comprising a capacitor and an RF switch, wherein switching the plurality of switch capacitor networks changes the capacitance of the resonant coil, thereby changing the resonant frequency of the resonant coil. The near field RFID probe further includes a probe control module configured to adjust the resonant frequency of the resonant coil to maintain the predetermined resonant frequency by switching the switch capacitor networks responsive to detecting that the resonant frequency of the resonant coil has deviated from the predetermined resonant frequency.

RELATED APPLICATIONS

This application claims priority from U.S. patent application Ser. No.16/230,266, filed 21 Dec. 2018, which is incorporated herein in itsentirety.

TECHNICAL FIELD

This disclosure relates to Radio Frequency Identification (“RFID”)tracking and, more particularly, to a tunable near field RFID probe.

BACKGROUND

Radio Frequency Identification (“RFID”) is commonly used for trackingand identifying items, such as electronics, as the items are transportedand moved through a supply chain, for example, or when items aredeployed for use in the field. A use of a portable near field readerallows for rapid identification and validation of the items. While theterm “near field” is commonly used to refer to a standard operating at13.56 MHz, the term “near field” as used herein may refer to any systemwhere the distance between a transmitter and a receiver is small (i.e.under ½π wavelengths), and where antennas are replaced by pick up coilssuch that magnetic coupling dominates.

SUMMARY

Described herein is a near field link operating at microwave frequencies(i.e. in GHz) over short distances measured in millimeters orcentimeters,

In one example, a radio-frequency identification (“RFID”) probe isprovided that comprises a probe tip comprising a resonant coilconfigured to communicate with an RFID compatible device at apredetermined resonant frequency, and a plurality of switch capacitornetworks each comprising a capacitor and an RF switch. The switching ofthe plurality of switch capacitor networks changes the capacitance ofthe resonant coil, thereby changing a resonant frequency of the resonantcoil. The RFID probe further comprises a probe control module configuredto adjust the resonant frequency of the resonant coil to maintain thepredetermined resonant frequency by switching the switch capacitornetworks responsive to detecting that the resonant frequency of theresonant coil has deviated from the predetermined resonant frequency.

In another example, a tunable resonant coil comprising a plurality ofswitch capacitor networks, each comprising a capacitor and an RF switch.The plurality of switch capacitor networks is configured to change thecapacitance of the resonant coil, thereby change the resonant frequencyof the resonant coil, responsive to receiving a control signalindicating that the resonant frequency of the resonant coil has deviatedfrom a target resonant frequency.

In yet another example, a method includes establishing communicationbetween a resonant coil of a radio-frequency identification (“RFID”)probe with an RFID tag at a predetermined resonant frequency. The methodfurther includes determining that the resonant frequency of the resonantcoil has deviated from the predetermined resonant frequency. The methodfurther includes adjusting the resonant frequency of the resonant coilby switching a switched capacitor network coupled to the resonant coil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a radio-frequency identification (RFID) probe withresonant frequency tuning capabilities.

FIG. 2 illustrates an example resonant coil of the RFID probe of FIG. 1.

FIG. 3 illustrates an example switch capacitor network of the resonantcoil of FIG. 2.

FIG. 4 illustrates a back side of an example substrate on which aresonant coil of FIG. 2 can be laid out.

FIG. 5 illustrates a method for tuning an RFID probe.

DETAILED DESCRIPTION

One example RFID application combines encryption, sensors, near-fieldpower, and communications into an integrated circuit referred to as a“dielet” that can be inserted into an item or into packaging of theitem. A dielet is a small tag (e.g. 100 u×100 u) with an on chip pickupcoil. In one example, the pickup coil is excited at a high microwavefrequency, such as of 5.8 GHz. RFID tracking is accomplished by using anexternal portable probe to communicate with RFID enabled devicesincluding the dielet. A tip of the probe and the dielet each include acoil to establish communication via an inductive coupling at apredetermined resonant frequency. The coils on both the dielet and theprobe tip are tuned to the predetermined resonant frequency in order tomaximize power transfer. However, the resonant frequency of the probetip may change as the environment changes, and therefore, deterioratingthe power transfer. Specifically, changes in the inductance between theprobe tip and the dielet, that result from changes in distance betweenthe probe tip and the dielet, shifts the resonant frequency. Moreover,metal objects in the local area near the dielet and the probe tip mayalso cause a shift in the resonant frequency.

This disclosure relates to a near field radio-frequency identification(RFID) probe with resonant frequency tuning capabilities to overcome theproblem of shifting resonant frequency. In particular, described hereinis a resonant frequency tuning system which uses RF switches andcapacitors in a tip of a probe to tune a resonant frequency of the probetip to a desired resonant frequency that matches a resonant frequency ofan RFID compatible device including an RFID tag, such as a dielet. Thus,power transfer deterioration between the probe tip and the RFID tagresulting from environment changes is reduced or eliminated.

FIG. 1 illustrates an example an RFID probe with resonant frequencytuning capabilities (hereinafter “probe”) 100. The probe 100 includes aprobe tip 102 at an end of the probe 100. The probe 100 may include anysuitable form or shape. In one example, the probe 100 is a narrow anelongated shape, such as a shape of a stylus pen, for enabling handlingand maneuvering of the probe tip 102. In one example, the probe tip 102may be removable and interchangeable.

The probe tip 102 includes a resonant coil 104 for establishing acommunication via an inductive coupling at a predetermined resonantfrequency with an external RFID tag (such as a dielet) 106 of an RFIDenabled device. In particular, the resonant coil 104 of the probe tip102 establishes communication with the RFID tag 106 upon the probe 100being maneuvered into a position such that the probe tip 102 isproximate to the RFID tag 106.

The probe 100 includes a probe reader 108 configured to facilitatecommunication with the RFID tag 106 via the inductive coupling formedbetween the RFID tag 106 and the resonant coil 104. In particular, theprobe reader 108 is electrically coupled to the probe tip 102 and isconfigured to produce and send RF signals to the probe tip 102 forcommunication to the RFID tag 106 via the resonant coil 104. The probereader 108 is also configured to receive and interpret RF signals fromthe RFID tag 106.

The probe reader 108 is further configured to control the resonantfrequency of the resonant coil 104. Thus, the probe reader 108 is ableto dynamically make adjustments to the resonant frequency of theresonant coil 104 in real time when the resonant frequency shifts, suchas when the environment changes. In particular, the probe reader 108 isconfigured to detect or determine the resonant frequency of the resonantcoil 104. In one example, the probe 100 includes a sensor 110 configuredto determine the resonant frequency of the resonant coil 104. The probereader is communicatively coupled to the sensor 110. Thus, the probereader 108 is configured to determine the resonant frequency of theresonant coil 104, and in particular a deviation in the resonantfrequency, by communicating with and receiving data from the sensor 110and analyzing the data received from the sensor 110. In one example, theprobe reader 108 continuously scans for and determines changes inresonant frequency. In another example, the probe reader 108periodically scans for and determines changes in resonant frequency.

Although the example probe 100 is illustrated to include the probereader 108, another example probe (not shown) may be configured tocommunicate with an external probe reader (not shown) via a cable suchas a coaxial cable. For example, a probe reader may be a mobilecomputing device such a laptop computer, a tablet computer, or asmartphone. It should be further appreciated that the probe reader 108,also referred to herein as a control module, may include electricalcircuitry, executable computer logic, or a module including acombination of electrical circuitry and executable computer logic.

It should also be appreciated that, although the sensor 110 and theprobe reader 108 are depicted as two components, the sensor 110 and theprobe reader 108 may also be combined into a single component or device.In one example, the probe 100 may include the sensor 110 and beconfigured to communicate with an external probe reader (not shown). Inanother example, both the probe reader 108 and the sensor 110 may beexternal to the probe 100.

The resonant coil 104 includes switches and capacitors (not shown) thatenable tuning of the resonant coil 104. In particular, addingcapacitance to the resonant coil 104 changes the resonant frequency ofthe resonant coil 104. Thus, by turning on or off a series of switchescoupled to a series of capacitors, the resonant frequency of theresonant coil 104 can be tuned. The probe reader 108 is configured tomake adjustments to the resonant frequency of the resonant coil 104, inresponse to determining that the resonant frequency of the resonant coil104 has shifted or deviated away from the target, predetermined, ordesired resonant frequency, by sending a DC voltage signal that controlsthe switches.

The probe reader's 108 control of the resonant frequency of the resonantcoil 104 via the activation or deactivation of capacitors will befurther understood and appreciated by looking at the resonant coil 104in more detail. FIG. 2 illustrates an example resonant coil 200 (e.g.corresponding to the resonant coil 104 of FIG. 1). The resonant coil 200is laid out in a crisscross spiral form with multiple strands 212, eachof which follows a similar path, allowing for equal current sharing andtherefore minimizing current loss. In the example resonant coil 200, themultiple strands 212 are connected in parallel with symmetric branchingpaths 206 a, 206 b, 206 c, and 206 d (hereinafter “branching path(s)206) for receiving an RF signal from a probe reader (e.g. correspondingto the probe reader 108 of FIG. 1). This reduces total inductance andincreases self-resonance of the resonant coil 200. In another exampleresonant coil (not shown), multiple strands may be connected in series.

In one example, the multiple strands 212 are each either fed withdifferential RF excitation or with single ended RF excitation, whereinthe far ends of each strand are grounded. In one example, the multiplestrands 212 are driven from a common RF signal brought to the probe tip102 through a single connection with RF splitters, baluns, and/ormatching networks on chip with the resonant coil 200. In one example,the multiple strands 212 are driven from a common RF signal and aplurality of step-down matching impedance networks to match the lowimpedance characteristic of a high Q series resonant network, in thisexample, the impedances of the various strands of the multiple strands212 are aggregated in parallel, thus reducing the step-down ratio neededto achieve a given Q.

It should be appreciated that for a series resonant network, the propermatching impedance equals the square root of (inductance/capacitance)/Q,as such a high Q network requires a low impedance driving impedance tomatch it. It should be further appreciated that the achievable bandwidthof a given matching structure is reduced as the impedance matching ratiois increased. Thus, it is advantageous to use a plurality of interleavedstrands and an associated plurality of matching networks when driving ahigh Q series resonant coil since the step-down ratio of each individualmatching network need not be as large, thereby allowing for the matchingnetworks to have a higher bandwidth than would otherwise be possible.

In one example, the multiple strands 212 are physically constructed withmultiple RF feeds (e.g. two, four, eight, etc.) located symmetricallyaround the perimeter of the resonant coil 200 to allow for equal currentdivision and matching parasitic capacitance among each of the strandsand also to allow for a well-balanced RF feed network. This in turnenables the plurality of matching networks to be symmetrically locatedor arranged around the resonant coil 200 as well, and also to have equalcharacteristics, thus allowing the multiple strands 212 to equally sharecurrent and minimize loss.

The resonant coil 200 includes four switch capacitor networks 202 a, 202b, 202 c, and 202 d (hereinafter “switch capacitor network(s) 202”).Although four switch capacitor networks 202 are illustrated, anotherexample resonant coil (not shown) may include any suitable number ofswitch capacitor networks. The switch capacitor networks 202 arepositioned symmetrically around the resonant coil 200, which providesfor equal phase signals between each of the switch capacitor networks202 from a single RF port via the branching paths 206. Thus, a singleelectrical connection from a probe reader (e.g. corresponding to theprobe reader 108 of FIG. 1). facilitates control of the resonant coil200 without the need for additional wires.

Each of the switch capacitor networks 202 include a switch 204 a, 204 b,204 c, and 204 d (hereinafter switch(es) 204). In one example, theswitch 204 is a Super-Lattice Castellated Field Effect Transistor(SLCFET), such as shown in US patent publication no. 2016/0293713. AnSLCFET switch is a low loss, high R_(on)*C_(off) switch, and thereforeenables the resonant coil 200 to be tuned while maintaining a highquality (Q) factor. It should be appreciated that other types ofswitches, such as a Gallium Arsenide (GaAs) switch, may be used. A DCvoltage control signal from the probe reader is routed to the respectivegates of the switches 204 to switch the switches 204 on and off.

The switch capacitor networks 202 each further include a first capacitor208 a, 208 b, 208 c, and 208 d (hereinafter “first capacitor 208”),respectively, coupled to the switch 204 in parallel. The switchcapacitor networks 202 each further include a second capacitor 210 a 210b, 210 c, and 210 d (hereinafter “second capacitor 210”), respectively,coupled to the switch 204 in series. Thus, depending on whether theswitch 204 is in an on or off state, either the first capacitor 208 orthe second capacitor 210 is charged. Since the resonant frequency of theresonant coil 200 is affected based on which of the first and secondcapacitors 208 and 210 are being charged, the resonant frequency of theresonant coil 200 may be adjusted by turning the switch 204 on or off.

By incorporating four slightly differently configured switch capacitornetworks 202 into the resonant coil 200, the resonant coil 200 may betuned to a desired resonant frequency based on various combinations ofactivated switches 204. In particular, the four switch capacitornetworks 202 each have three tuning states which can be selected byapplying a positive, ground, or negative DC bias voltage to the singleRF port in order to flip the appropriate switches 204. By configuringthe switches 204 to respond differently to the positive, ground, ornegative DC bias, the switch capacitor networks 202 may be engaged invarious combinations in order to tune the resonant coil 200 to a desiredresonant frequency.

For example, the first and second switches 204 a and 204 b may beconfigured to go to an ON state when a positive DC bias is applied, thethird and fourth switches 204 c and 204 d may be configured to go to anON state when a negative DC bias is applied, and all four switches 204 a204 b, 204 c, and 204 d may be configured to go to an ON state when theDC bias is ground. Thus, the different states of the switch capacitornetworks 202 allow for different combinations of capacitors to charge,thereby tuning the resonant frequency of the resonant coil 200 to adesired resonant frequency.

It should be appreciated that configurations described herein are notlimiting and that the switches 204 may be configured to respond to thepositive, ground, or negative DC bias in any suitable combination. Itshould be further appreciated that although the resonant coil 200 isillustrated as a differential form of a coil with four differentialinputs, in another example embodiment (not shown), a resonant coil mayinclude four single ended inputs with the far end of the of the coil'sturns grounded.

FIG. 3 illustrates one example configuration of a first switch 302 and asecond switch 304 of a switch capacitor network 300 (e.g. correspondingto the switch capacitor network 202 of FIG. 2). In particular, the firstswitch 302 is coupled to a resistor R1 at a gate and coupled toresistors R2 and R3 at a drain and source such that the first switch 302is ON when a positive DC bias 306 is applied. The second switch 304 iscoupled to resistor R4 and R5 and a drain and source and coupled toground at a gate such that the second switch 304 is ON when a negativeDC bias 306 is applied. In addition, the first switch 302 and the secondswitch are both configured to be ON when the DC bias 306 is ground. Itshould be appreciated that although two-bit tuning control is describedherein, another example embodiment (not shown) may include one-bittuning.

In one example, resistors R1, R2, R3, R4, and R5 could be large(approximately 100 kOhm, for example) in order to avoid impacting Q. Inone example, tuning capacitors C1, C2, C3, and C4 may be a smallpercentage of primary resonance capacitors C5 and C6. The switchcapacitor network 300 may further include diodes (not shown) in order toavoid excessive forward bias of the first and second switches 302 and304.

In one example, a Litz wire style coil winding may be used to increasemetal cross section which current flows through in order to improve Q.Moreover, although the example illustrated herein depict a 4-turn coil,an 8-turn coil may also be used in order to further improve Q.

In one example, a resonant coil (e.g. corresponding to the resonant coil104 of FIG. 1 or the resonant coil 200 of FIG. 2) is laid out on asubstrate such as a GaN substrate. In one example, the substrate alsoincludes a slotted ground plane on an exposed surface in order to reduceeddy currents while allowing a magnetic field. FIG. 4 illustrates a backside 402 of an example substrate 400 including a slotted ground plane404. The resonant coil is disposed on an inside surface (not shown) orfront of the substrate 400 for added protection. The substrate 400 isconfigured to be disposed on probe tip (e.g. corresponding to the probetip 102 of FIG. 1) such that the coil side or the inside surface facesinward and the back side 402 is exposed and faces outward.

An RFID probe (e.g. corresponding to the probe 100 of FIG. 1) will befurther appreciated with reference to a method for tuning the probeillustrated in FIG. 5. At block 502, communication is establishedbetween a resonant coil of an RFID probe and an RFID tag at apredetermined resonant frequency by bringing the RFID probe intoproximity of the RFID tag. At block 504, The resonant frequency of theresonant coil is measured. The deviation may be determined, for example,by a sensor either embedded in the RFID probe or external to the RFIDprobe. At block 506, a determination is made whether the resonantfrequency of the resonant coil has deviated away from a target ordesired resonant frequency.

If, at block 506, a determination is made that the resonant frequency ofthe resonant coil has not deviated from the desired resonant frequency,the RFID TAG is read by the RFID probe tip at block 510. If, at block506, a determination is made that the resonant frequency of the resonantcoil has deviated from the desired resonant frequency, a determinationis then made, at block 508, whether the deviation is greater than apredefined threshold amount. In one example, the threshold amount may bedetermined based on a percentage of the desired resonant frequency. Forexample, a threshold amount may equal a 1% change in value of theresonant frequency.

If, at block 508, a determination is made that the deviation is notgreater than the predefined threshold amount, than the RFID TAG is readby the RFID probe tip at block 510. If, at block 508, a determination ismade that the deviation is greater than the predefined threshold amount,then, at block 512, the resonant frequency of the resonant coil isadjusted based on the determined deviation. The adjustment may be made,for example, by a control module appropriately switching switchcapacitor networks coupled to the resonant coil. Once the adjustment ismade, the resonant frequency of the resonant coil may be again measuredat block 504. In one example, rather than again measuring the resonantcoil at block 504, the RFID TAG is read by the RFID probe tip at block510 after the adjustment is made at block 512.

What have been described above are examples. It is, of course, notpossible to describe every conceivable combination of components ormethodologies, but one of ordinary skill in the art will recognize thatmany further combinations and permutations are possible. Accordingly,the disclosure is intended to embrace all such alterations,modifications, and variations that fall within the scope of thisapplication, including the appended claims. As used herein, the term“includes” means includes but not limited to, the term “including” meansincluding but not limited to. Additionally, where the disclosure orclaims recite “a,” “an,” “a first,” or “another” element, or theequivalent thereof, it should be interpreted to include one or more thanone such element, neither requiring nor excluding two or more suchelements.

What is claimed is:
 1. A radio-frequency identification (“RFID”) probecomprising: a probe tip comprising a resonant coil configured tocommunicate with an RFID compatible device at a predetermined resonantfrequency; a plurality of switch capacitor networks each comprising acapacitor and an RF switch, wherein switching the plurality of switchcapacitor networks changes the capacitance of the resonant coil, therebychanging a resonant frequency of the resonant coil; a probe controlmodule configured to adjust the resonant frequency of the resonant coilto maintain the predetermined resonant frequency by switching the switchcapacitor networks responsive to detecting that the resonant frequencyof the resonant coil has deviated from the predetermined resonantfrequency; and a substrate disposed in the probe tip, the substratecomprising a slotted ground plane on an exposed surface to reduce eddycurrents from canceling out projected magnetic fields.
 2. The RFID probeof claim 1, further comprising a sensor configured to determine theresonant frequency of the resonant coil, wherein the probe controlmodule is communicatively coupled to the sensor and configured to detectthe deviation in the resonant frequency by analyzing sensor datareceived from the sensor.
 3. The RFID probe of claim 1, wherein theprobe control module is configured to switch the plurality of switchcapacitor networks by sending a DC voltage signal to the plurality ofswitch capacitor networks.
 4. The RFID probe of claim 3, wherein theprobe control module is configured to send one of a positive DC biasvoltage signal, a negative DC bias voltage signal, and a ground DC biasvoltage signal, wherein DC voltage is received on a same connection asan RF signal, and wherein each of the RF switches of the plurality ofswitch capacitor networks are configured to respond to the DC voltagesignal to provide different comparative values to adjust the resonantfrequency of the resonant coil.
 5. The RFID probe of claim 1, whereinthe RF switches comprise a Super-Lattice Castellated Field EffectTransistor (SLCFET).
 6. The RFID probe of claim 1, wherein the resonantcoil and the plurality of switch capacitor networks are located in aninside surface of the substrate.
 7. The RFID probe of claim 1, whereinthe resonant coil comprises a plurality of strands in a crisscrossspiral form that are connected in parallel.
 8. The RFID probe of claim7, wherein the plurality of strands are fed with one of differential RFexcitation and single ended RF excitation, and wherein the far end ofeach of the plurality of strands are grounded.
 9. The RFID probe ofclaim 7, wherein the plurality of strands are driven from a common RFsignal brought to the probe tip through a single connection with atleast one of RF splitters, baluns, and on chip matching networks withthe resonant coil.
 10. The RFID probe of claim 7, wherein the pluralityof strands are driven from a common RF signal and a plurality ofstep-down matching impedance networks, wherein the impedance of theplurality of strands matches a low impedance characteristic of a high Qseries resonant network, and wherein the impedances of each of thestrands of the plurality of strands are aggregated in parallel.
 11. TheRFID probe of claim 7, wherein the plurality of strands are constructedwith a plurality of RF feeds located symmetrically around a perimeter ofthe resonant coil.
 12. A tunable resonant coil comprising: a pluralityof strands in a crisscross spiral form that are connected in parallel;and a plurality of switch capacitor networks, each comprising acapacitor and an RF switch, wherein the plurality of switch capacitornetworks are configured to change a capacitance of the resonant coil,thereby change a resonant frequency of the resonant coil, responsive toreceiving a control signal indicating that the resonant frequency of theresonant coil has deviated from a predetermined resonant frequency,wherein the plurality of switch capacitor networks are configured tochange the capacitance of the resonant coil by receiving a DC voltagesignal comprising one of a positive DC bias voltage signal, a negativeDC bias voltge signal, and a ground DC bias voltage signal.
 13. Thetunable resonant coil of claim 12, further comprising: a sensorconfigured to determine the resonant frequency of the resonant coil; anda control module coupled to the sensor configured to communicate thecontrol signal; wherein the control module is configured to communicatethe control signal responsive to detecting the deviation in the resonantfrequency by analyzing sensor data received from the sensor.
 14. Thetunable resonant coil of claim 12, wherein each of the RF switches ofthe plurality of switch capacitor networks are configured to respond tothe DC voltage signal to provide different comparative values to adjustthe resonant frequency of the resonant coil.
 15. The tunable resonantcoil of claim 12, wherein the RF switches comprise a Super-LatticeCastellated Field Effect Transistor (SLCFET).
 16. The tunable resonantcoil of claim 12, wherein the tunable resonant coil is disposed on aninside surface of a substrate, and wherein the substrate comprises aslotted ground plane on an exposed surface.
 17. The tunable resonantcoil of claim 12, wherein the tunable resonant coil is disposed in aradio-frequency identification (“RFID”) probe.
 18. The tunable resonantcoil of claim 12, wherein the tunable resonant coil is configured toreceive the control signal from a control module responsive to thecontrol module detecting the deviation in the resonant frequency byanalyzing sensor data received from a sensor.
 19. A tunable resonantcoil comprising: a plurality of switch capacitor networks, eachcomprising a capacitor and an RF switch, wherein the plurality of switchcapacitor networks are configured to change a capacitance of theresonant coil, thereby change a resonant frequency of the resonant coil,responsive to receiving a control signal indicating that the resonantfrequency of the resonant coil has deviated from a predeterminedresonant frequency, wherein the tunable resonant coil is disposed in aradio-frequency identification (“RFID”) probe, wherein the plurality ofswitch capacitor networks are configured to change the capacitance ofthe resonant coil by receiving a DC voltage signal comprising one of apositive DC bias voltage signal, a negative DC bias voltage signal, anda ground DC bias voltage signal.